Electrical discharge machining pulse control method and apparatus

ABSTRACT

A system for controlling machining pulse on-off time to limit gap power in response to a short or an open circuit machining gap condition in electrical machining systems wherein a multivibrator applies a preset on-off time at an electronic switch between the power supply and the gap. The condition detected at the gap in the form of an electrical signal is delivered via a delay network to a control circuit adapted to shunt one of the impedances of the resistance-capacitance network of the multivibrator.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of my U.S. application Ser.No. 493,473, filed Oct. 6, 1965, and entitled "Solid-State PulseGenerator for Electric-Discharge Machining," now U.S. Pat. No. 3,360,683issued Dec. 26, 1967.

BACKGROUND OF THE INVENTION

The field to which my invention relates is that generally denoted aselectrical machining. Included in this field are electrical dischargemachining, electrochemical machining and electrochemical dischargemachining in which processes metal removal is accomplished by passing ofdiscrete electrical impulses across a coolant filled gap. In electricaldischarge machining, the coolant is a dielectric fluid such as keroseneor transformer oil. The process and apparatus of electrical dischargemachining are explained and shown in my U.S. Pat. No. 3,054,931, issuedSept. 18, 1962 and entitled "Electric Power Supply Apparatus forElectric Discharge Machining." The process of electrochemical machiningis explained and shown in my U.S. application Ser. No. 316,955, filed onOct. 17, 1963 (now U.S. Pat. No. 3,357,912, issued Dec. 12, 1966) whilethat of electrochemical discharge machining is disclosed in myco-pending U.S. application Ser. No. 475,375, filed on July 28, 1965.Iadd.(now U.S. Pat. No. 3,616,343).Iaddend.. In each of theseelectrical machining processes, an electrode servo feed system isemployed to maintain an optimum gap spacing between electrode andworkpiece. A liquid coolant in the form of a dielectric or electrolyte,depending on the process used, is circulated through the gapcontinuously during the machining operation. If the initial downfeed ismade with too high a power pulse input to the gap, damage can be causedto workpiece, electrode or both. Provision is made by the presentinvention to provide for current reduction by concomitantly orseparately increasing pulse off-time and decreasing pulse on-time duringgap open circuit. When the gap becomes too narrow or is bridged orcontaminated by eroded particles, an abnormal condition called "gapshort circuit" can arise. My invention is effective in controlling allthe foregoing types of electrical machining where a succession of powerpulses is utilized and the variation in dimension and condition of themachining gap occurs from time to time. While the present invention isdescribed in terms of transistor switch circuitry, my invention is notso limited, but is equally applicable to any electronic switcharrangement. By " electronc switch" I mean any electronic control devicehaving three or more electrodes comprising at least two principal orpower electrodes acting to control current flow in the power circuit,the conductivity between the principal electrodes being determined by acontrol electrode within the switch whereby the conductivity of thepower circuit is regulated statically or electrically without movementof mechanical elements within the switch. Included within thisdefinition are vacuum tubes and transistors in which turn-on isaccomplished by a control voltage applied to the control electrode andin which turn-off is accomplished automatically in response to theremoval of that control voltage. Also included in the definition aredevices of the gate type in which turn-on is accomplished by a controlvoltage applied to the control electrode which control voltage may bethen removed and in which turn-off is accomplished by application of asubsequent control voltage to the control electrode. An additional classof electronic switches called electronic trigger devices falls withinthis definition and includes ignitrons, thyratrons and semiconductorcontrolled rectifiers. By "electronic trigger device" I mean anyelectronic switch of the type which is triggered on at its controlelectrode by a pulse and is turned off by reverse voltage applied for asufficient time across its principal electrodes.

SUMMARY OF THE INVENTION

My invention provides a feedback and control circuit which regulates themachining power pulses with respect to their on-time and off-time. Thecontrol is exercised over the pulse generator after the imposition of asuitable delay interval.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a pulse generator suitable forelectrical machining;

FIG. 2 is a schematic of a circuit usable with the circuit of FIG. 1according to the present invention for controlling machining-pulseon-time;

FIG. 3 is a schematic of a circuit usable with the circuit of FIG. 1according to the present invention for controlling machining-pulseoff-time; and

FIG. 4 is a somewhat more detailed schematic of a pulse generatorsubstantially similar to that of FIG. 1 including provision for gap opencircuit pulse control.

DESCRIPTION

In FIG. 1, the basic elements of a pulse generator for an electricalmachining system are shown. The machining gap lies between electrode 14and workpiece 15 and is flooded with a coolant fluid during machining. Aservo feed system, not shown, is employed to maintain substantiallyconstant gap spacing as workpiece material is removed. A polarityreversal switch 30 is included. A power supply MPS is connected inseries with the gap and the principal or power-conducting electrodes ofa plurality of parallel connected switching NPN transistors A₁₀, A₁₂,A₁₃. The switching-transistor bank 32 may include as many transistors asare needed to meet the machining-power-output requirements. Thetransistors included in switching bank 32 are triggered on and off bythe operation of an amplifier transistor AT. The power supply fortransistor AT is shown at APS.

The machining-pulse on-time, off-time and frequency are preset andcontrolled through an astable multivibrator 34 including two alternatelyconductive transistors MV₁ and MV₂. The power supply for themultivibrator is designated as TPS. Adjustable RC networks are connectedbetween the two transistors MV₁ and MV₂ as shown. These networks includeresistors R₁, R₂ and capacitors C₁, C₂. The RC network R₁ C₁ coupled totransistor MV₁ determines the basic multivibrator pulse on-time inaccordance with the formula T_(on) =KC₁ R₁. The RC network R₂ C₂connected to transistor MV₂ determines pulse off-time in accordance withthe formula T_(off) =KC₂ R₂ where K, as in the above formula, representsa circuit constant given by the design of the multivibrator. Collectorresistors MV₆ and MV₇ are connected as indicated. By ganged operation ofthe variable resistor-capacitor components, it is possible to vary thepulse on-off time with or without altering the frequency. The frequencyof the multivibrator output pulses is determined by the formula

    F=1/K(R.sub.1 C.sub.1 +R.sub.2 C.sub.2)

a variable-resistance network, including fixed resistor LR₂ andpotentiometer LR₁ in series arrangement is connected across themachining gap. It is the function of this network to provide a signalproportional to average gap voltage through terminals E-F for pulsefeedback control as will be explained in connection with FIGS. 2 and 3hereinafter. While the present invention involves the use of a gapvoltage signal, it is possible to use gap current or power as the signalrelied upon to indicate abnormal gap condition.

FIG. 2 shows one embodiment of my invention adapted to perform machiningpulse on-time control independently of pulse off-time. Feedbackterminals E-F are connectible to like lettered terminals of the FIG. 1circuit. Terminals V, W and Z at the left-hand side of the drawing areconnectible to like designated terminals of FIG. 1. The several stagesof the feedback system include delay network 36, differentiating network38, integrating network 40 and sensing resistor SR. The final andleft-hand stage of the FIG. 2 circuit comprises a phase-reversingnetwork 42. Delay network 36 includes a series-connected adjustableinductor FD₁ and a plurality of capacitors FD₂ coupled in a delay-lineconfiguration with the inductor. By way of example, the delay time forelectrical-discharge machining may range between 1 to 500 microsecondswhile a somewhat shorter delay time of from 1 to 50 microseconds may beprovided for electrochemical machining. When the machining gap betweenelectrode 14 and workpiece 15 is in normal cutting or in open circuitcondition, an integrated voltage which is proportional to mean gapvoltage will exist across sensing resistor SR with the polarityindicator. When short circuit or arcing occurs at the machining gap, theintegrated voltage will rapidly drop in magnitude or return to zero.Phase reversing network 42 includes NPN transistor FT₁ having its baseconnected through a limiting resistor to the upper terminal of resistorSR. A second NPN transistor FT₂ is coupled between transistor FT₁ and aPNP transistor Tr as shown. Transistor Tr, when in its conductive state,places its internal resistance r across the terminals V-W to provide atotal resistance between V-W of ##EQU1## where R is the resistance ofpotentiometer R₁.

In the operation of the circuit, terminals V, W and X are interconnectedas between FIGS. 1 and 2. A voltage appears across sensing resistor SRwith the polarity shown, which voltage is proportional to the mean gapvoltage. If this voltage is of a level sufficient to indicate normal gapcutting or gap open circuit condition, transistor FT₁ will be turned onwith conduction from power supply TPS occurring through itscollector-emitter electrodes. Transistors FT₂ and channelling transistorTr will both be rendered non-conductive.

In the other condition, when voltage across sensing resistor SR hasfallen to zero or drastically dropped responsive to gap short circuit orarcing condition, transistor FT₁ will be turned off. This will rendertransistor FT₂ conductive through power supply TPS whereby channellingtransistor Tr becomes conductive with the resultant effect of itsinternal resistance r.

FIG. 3 represents another embodiment of the feedback control system alsoconnectible to the FIG. 1 circuit. This embodiment relates to off-timecontrol of the multivibrator and of the machining power pulses. Theseveral stages of the feedback system are like those of the FIG. 2circuit and are similarly designated. The off-control system of the FIG.3 circuit, however, does not include phase reversing network 42. Thechannelling transistor is designated as Tr' with its internal resistancer'. Terminals R-S are connectible to the like designated terminals ofFIG. 1. Responsive to the magnitude of the voltage developed acrosssensing resistor SR, transistor Tr' will be maintained conductive ornon-conductive to control the multivibrator off-time duration.

FIG. 4 shows a still further embodiment of my invention. This embodimentrelates to machining pulse on-off time control responsive to gap opencircuit condition which may be combined with the short circuit controlsystems of FIGS. 2 and 3. The transistorized pulse generator is shown insomewhat more complete detail than in the drawing of FIG. 1. Machiningpower supply MPS receives a three-phase alternating current input Ithrough a switch I₁. Included in the input lines is a three-phasesaturable core reactor I₂ whose control voltage is furnished by acontrol unit I₃ having input lines I₄. The main power supply MPScomprises a three-phase step-down transformer MPS₁ and a full-waverectifier MPS₂ whose negative output terminal is grounded while thepositive terminal is connected through the smoothing choke MPS₃ andfilter capacitors MPS₄ to workpiece 15 located within workpan 16.Electrode 14 is juxtaposed with workpiece 15 while a flow of dielectricis maintained through the gap by a circulation system as is well knownto the art.

The power supply TPS for the multivibrator includes a single-phasetransformer TPS and a rectifier bridge TPS₂ whose negative side isgrounded while its positive terminal is coupled to transistor lead +Athrough filter choke TPS₄ and a pair of filter capacitors TPS₃ with theoutput lead being designated TPS₅.

The solid-state multivibrator MV for triggering the switchingtransistors comprises a pair of NPN transistors MV₁, MV₂ whose emittersare connected to ground through bias resistors MV₃ and MV₄, the groundlead being shown as MV₅. The high voltage side of transistor powersupply TPS is applied to the multivibrator transistors through collectorresistors MV₆ and MV₇ while cross coupled RC circuits comprising R₁ C₁and R₂ C₂ serve to control the conduction time of the multivibrator MVin the manner already set forth in connection with the circuit of FIG.1.

The pulse output train from the multivibrator is taken from output lineMV₁₃ and applied to the main signal line A_(I) of a plurality ofelectronic switching circuits A_(I), A_(II), A_(III) . . . A_(n), theactual number being determined in accordance with the number ofswitching transistors per circuit, the individual current carryingcapacity of each transistor, and the total current to be deliveredduring the machining pulses.

Each of the electronic switch circuits A_(I), A_(II) . . . A_(n)comprises a multiplicity of switching transistors A₁₀, A₁₁, A₁₂ whoseemitter-collector electrodes are connected in parallel across leads A₇and A₈, the former being connected to ground via lead MV₅ while thelatter is connected to electrode 14 through diode 10, ammeter 11 andvariable resistor 12. The collector-emitter electrodes of transistorsA₁₀, A₁₁, A₁₂ are thus connected in series between ground and electrode14 while the machining current is applied to workpiece 15 by the line+B. Each of transistors A₁₀, A₁₁, A₁₂ of each switching circuit A_(I),A_(II), A_(III) . . . A_(n) has its control electrode, i.e., base inseries circuit with a resistor A₂₁, A₂₂, A₂₃, respectively, andconnected to a signal lead A₉ to which the output terminal A₆ of arespective amplifier transistor AT is connected. Each of the transistorsAT has its base resistor A₃ connected to the signal line A₁. Thecollectors of the transistors AT may be connected through bias resistorsA₄ to the high voltage side of the transistor power supply TPS alongline +A or to the plus positive side of a separate power supply such asAPS shown in the circuit of FIG. 1.

In the FIG. 4 circuit, machining power is controlled by detecting thefrequency of discharge by a frequency meter or other means responsive tothe repetition rate of gap discharges. Since the repetition rate isproportional to the mean current, it is possible to use the latter asthe measure of frequency for control of the main power supply MPS. Forthis purpose, a current transformer I₅ can be connected in one branch ofthe transistor switching system so that it need have only the capacityof that branch. Current transformer I₅ has its terminals I₄ connectedwith control unit I₃ in the upper left-hand side of the drawing. Controlunit I₃ generates the direct current control voltage applied tosaturable reactor I₂. Thus, the dicharge power may be maintainedconstant by control unit I₃ which compares the output of currenttransformer I₅ with an adjustable reference to generate the controlvoltage for saturable reactor I₂. Another type of control is possible byvarying the resistance of the series circuit in which electrode 14 isconnected. For this purpose, tapped resistor 12 is provided, while aswitch 13 is designed to selectively shunt the resistor sections ascontrolled by a rotary solenoid I₆ energized by current transformer I₅.As the current increases beyond the desired level, solenoid I₆ isenergized to shunt less of resistor 12.

The feedback control networks of the FIG. 4 circuit include delaynetwork 36 which includes capacitor FD₂ and variable inductor FD₁ in anarrangement like that shown in the FIGS. 2 and 3 circuits. Coupledacross the output of differentiating network 38 is the seriescombination of resistor F₄ and adjustable inductor F₅. The integratingnetwork 40 includes diodes F₆, F₇ and capacitors F₈, F₉ with the seriescombination of diode F₆ and capacitor F₈ connected across resistor F₄and the series combination of diode F₇ and capacitor F₉ connected acrossinductor F₅ in the manner shown. An intermediate capacitor F₁₀ isconnected as shown and shunted by sensing resistor F₁. A pair a PNPtransistors F₁₂ and F₁₄ are coupled to the multivibrator resistors R₁and R₂, respectively, for control of pulse on-off time in a mannersimilar to that previously indicated in connection with FIGS. 2 and 3.Current limiting resistors F₁₃, F₁₅ are connected to the bases oftransistors F₁₂ and F₁₄ as shown.

In the operation of FIG. 4 circuit, the voltage appearing acrosscapacitor F₈ in the integrating network indicates the level of mean gapvoltage and is represented by the formula RI where R designates theresistance of resistor F₄ and I designates current level of thedifferentiated signal of transformer 38. The voltage appearing acrosscapacitor F₉ is represented by the formula nLI where n denotes thefrequency of the differentiated input signal; L denotes the magnitude ofinductance and I denotes the current level of the differentiated signal.In this connection, it must be stated that in electrical-dischargemachining, to which this embodiment is particularly related, normal gapcutting involves the occurrence of harmonics of relatively highfrequency. Thus, when the gap is held in normal cutting condition, thevoltage nLI developed across inductor F₅ will be raised to higher levelwith the occurrence of higher frequency of acceptable dischargeharmonics. In the gap open circuit condition, the voltage will drop to areduced level will impression across the machining gap of a successionor train of pulses of a frequency determined by the setting of themultivibrator. Now the voltage difference between RI and nLI will bereflected across intermediate capacitor F₁₀ and across sensing resistorF₁. The integrating network of the FIG. 4 circuit is so designated thatsensing resistor F₁ has a polarity as indicated with the voltagedifference (nLI-RI) only when the gap is in the normal cutting conditionwhen sparks are occurring with harmonics of frequencies substantiallyhigher than the frequency of the multivibrator pulse train output. Thisis accomplished by presetting the relative magnitudes of the variableresistance R of F₄ and the variable inductance L of F₅. When the gapchanges to the open circuit condition, the voltage signal is developedacross sensing resistor F₁ with a different polarity (RI-nLI). In gapshort circuit condition, no voltage signal would be created acrossresistor F₁. In connection the terminals of sensing resistor F₁ withchannelling transistors F₁₂ and/or F₁₄, consideration must be given tothe inclusion of a phase reversal network. When pulse on-time narrowingin short circuit condition as well as in gap open circuit condition isdesired, a phase reversing network such as network 42 of FIG. 2 must beincorporated between the output of resistor F₁ and transistor F₁₄ in themanner shown in that figure. Off-time widening can be accomplished bydirect connection of the terminals of resistor F₁ with the base andemitter electrodes of transistor F₁₂ as shown in FIG. 4. It is similarlypossible to provide simultaneous on-time narrowing and off-time wideningwith or without frequency control. The present invention is not limitedto the single type of phase-reversing network shown, but may include anyconventionally known type of phase reversing network.

DESCRIPTION OF OPERATION

A description of operation will now be made with particular reference toFIGS. 1 and 2. A tabular representation of the circuit operationconditions will be helpful in the understanding of the presentinvention. When terminals V, W, X of FIG. 2 are connected with likelettered terminals of FIG. 1, respectively, the circuit will provideon-time control of the multivibrator and of the machining pulsesfurnished to the gap in response to electrical conditions of themachining gap without any effect of the off-time duration as shown inTable I below.

                                      TABLE I                                     __________________________________________________________________________                    Normal or open gap                                                                         Short circuit                                                    condition    condition                                        __________________________________________________________________________    Voltage level across resistor SR                                                              V            O.                                               Transistor FT.sub.1                                                                           Conductive   Non-conductive.                                  Transistor FT.sub.2                                                                           Non-conductive                                                                             Conductive                                       Transistor Tr   Non-conductive                                                                             Conductive (r).                                  Total resistance across R.sub.1                                                               R.sub.1 > R.sub.1 r/(R.sub.1 + r)                             Total resistance across R.sub.2                                                               R.sub.2  =R.sub.2                                             On-duration of output pulses                                                                  kC.sub.1 R.sub.1 > KC.sub.1 R.sub.1 r/R.sub.1 + r)            Off-duration of output pulses                                                                 kC.sub.2 R.sub.2 = kC.sub.2 R.sub.2                           Repetition rate of output pulses                                                              1/k(C.sub.2 R.sub.1 + C.sub.2 R.sub.2) < 1/k(C.sub.1                          R.sub.1 r/(R.sub.1 + r) + C.sub.2 R.sub.2)                    __________________________________________________________________________

As is apparent from Table I, pulse narrowing is carried out when themachining gap is approaching a short circuit or arcing condition. Thisis accomplished without change of the pulse off-time and withcorresponding increase of pulse frequency.

When terminals R and S of FIG. 3 are connected to like letteredterminals of FIG. 1, respectively, operation in the reverse mode to thatabove described will be effected. Off-time will be controlled as shownbelow in Table II.

                                      TABLE II                                    __________________________________________________________________________                    Normal or open gap                                                                         Short circuit                                                    condition    condition                                        __________________________________________________________________________    Voltage level across resistor SR                                                              V            O.                                               Transistor Tr   Conductive (r')                                                                            Non-conductive                                   Total resistance across R.sub.1                                                               R.sub.1=R.sub.1                                               Total resistance across R.sub.2                                                               R.sub.2 r'/(R.sub.2 +r')<R.sub.2                              On-duratin of output pulses                                                                   kC.sub.1 R.sub.1 =kC.sub.1 R.sub.1                            Off-duration of output pulses                                                                 kC.sub.2 R.sub.2 r'/(R.sub.2 +r')<kC.sub.2 R.sub.2            Repetition rate of output pulses                                                              1/k(C.sub.1 R.sub.1 +C.sub.2 R.sub.2 r ')>1k(C.sub.1                          R.sub.1 +C.sub.2 R.sub.2)                                     __________________________________________________________________________

In the embodiment of FIG. 3, the basic on-off time ratio and frequencyare determined by presetting the multivibrator parameters--R₁, R₂, C₁and C₂ together with the selection of the internal resistance r' of thetransistor Tr'. It is further possible to combine the two arrangementsshown above in connection with FIGS. 1, 2, and 3 to effect simultaneouson-time narrowing and off-time widening, with or without substantialchange of frequency. It is further possible to control only therepetition rate while maintaining a constant on-off ratio.

I claim:
 1. In an apparatus for machining a conductive workpiece bypassing electrical-discharge pulses between a tool electrode and saidworkpiece across a dielectric-coolant filled gap, a machining powercircuit for supplying said pulses to said gap, said machining powercircuit comprising a power supply; a first electronic switch having apair of principal electrodes connected in series with said power supplyand said gap, and a control electrode; a multivibrator having its outputconnected to the control electrode of said first switch for operating itwith predetermined on-off time, said multivibrator including a pair ofsecond electronic switches biased and coupled for alternate operationwhile having respective control electrodes and at least oneresistance-capacitance network connected to the control electrode of oneof said pair of said second switches for controlling said on-off time,said network having a time-constant-determining resistance; sensingmeans connected to said gap for providing an electrical output signalresponsive to gap short circuit condition; control means for alteringthe resistance of said network to decrease switch on-time responsive tosaid signal; and delay means coupled between said sensing means and saidcontrol means for delaying its operation a predetermined time intervalafter occurrence of said condition, said control means comprising athird electronic switch having its principal electrodes connected inshunt with said time-constant-determining resistance of said network forvarying the effective magnitude of said time constant and a controlelectrode connected to said gap through said delay means.
 2. In anapparatus for machining a conductive workpiece by passingelectrical-discharge pulses between a tool electrode and the workpieceacross a dielectric-coolant-filled gap, comprising a machining-powercircuit for supplying said pulses to said gap comprising a power supply,and a periodically operated electronic switch of preset on-off timehaving its principal electrodes connected in series with said powersupply and said gap for providing said pulses thereto and having acontrol electrode, the improvement which comprises sensing meansconnected to said gap for providing an electrical output signalresponsive to gap open circuit condition, control means operativelyconnected to the control electrode of said switch for decreasing itson-time responsive to said signal, and delay means coupled between saidsensing means and said control means for delaying its operation apredetermined time interval after occurrence of said condition.
 3. Theimprovement defined in claim 2 wherein:said sensing means includes avoltage-dividing resistor connected across said gap; said delay meansincludes a delay line having an inductance with one terminal connectedto said voltage-dividing resistor and a multiplicity of capacitorshaving corresponding terminals connected to said voltage-dividingresistor and tapped to said inductance; and said control means includesa differentiating network connected across another terminal of saidinductance and said corresponding terminals of said capacitors, and atleast one amplifying transistor connected between said differentiatingnetwork and said control electrode.
 4. The improvement defined in claim3, further comprising an integrating network connected between saiddifferentiating network and said amplifying transistor.
 5. Theimprovement defined in claim 4 wherein said periodically operatedelectronic switch comprises a plurality of parallel-triggered switchingtransistors each connected to said gap through a respective group ofpower transistors.
 6. The improvement set forth in claim 2 in which saidsensing means comprises a variable inductance for providing an outputsignal which is a function of relatively high frequency dischargesoccurring across the gap during normal cutting.
 7. In an apparatus formachining a conductive workpiece by passing electrical-discharge pulsesbetween a tool electrode and said workpiece across adielectric-coolant-filled gap, a machining power circuit for supplyingsaid pulses to said gap, said machining power circuit comprising a powersupply, a first electronic switch having a pair of principal electrodesconnected between said power supply and said gap and a controlelectrode, a multivibrator having its output connected to the controlelectrode of said first switch for operating it with predeterminedon-off time, said multivibrator including a pair of second electronicswitches biased and coupled for alternate operation, each of said pairof second switches having a respective control electrode connected to adifferent resistor-capacitor network, sensing means connected to saidgap for providing an electrical output signal responsive to gap shortcircuit condition, a first control means connected between said sensingmeans and one of said resistor-capacitor networks for changing its timeconstant and decreasing pulse on-time, a second control means connectedbetween said sensing means and the other of said resistor-capacitornetworks for changing its time constant and increasing pulse off-time,and delay means connected between said sensing means and each of saidcontrol means for controlling its operation a predetermined timeinterval after occurrence of said condition.
 8. The method of electricaldischarge machining comprising the steps of periodically connecting apower supply to a machining gap through an electronic switch to providemachining pulses of a predetermined on-off time and frequency thereto,wherein the improvement comprises sensing for the presence and absenceof substantially higher frequency discharges across said gap, providinga control output signal responsive to the absence of said high frequencydischarges, such condition being representative of gap open circuit andemploying said signal to increase the off-time and decrease the on-timeof said machining pulses.
 9. In an apparatus for machining a conductiveworkpiece by passing electrical pulses between a tool electrode and aworkpiece across a liquid-filled gap, the improvement which comprises:apower supply; periodically operable electronic switch means having atleast one control electrode and at least one pair of principalelectrodes in series between said power supply and said gap forsupplying said pulses thereto; periodically operable multivibrator meanshaving an output connected to said control electrode for triggering saidswitch means with preset on-off time, said multivibrator means includingat least one resistance-capacitance network having atime-constant-determining impedance for controlling on-off time; sensingmeans connected across said gap for providing an electrical outputsignal responsive to a condition of said gap; control means between saidsensing means and said impedance and bridged thereacross for alteringthe effective impedance value and the time constant of saidresistance-capacitance network in accordance with said signal; and delaymeans between said sensing means and said control means for delayingoperation of said control means for a predetermined time interval afteroccurrence of said condition.
 10. In an apparatus for machining aconductive workpiece by passing electrical pulses between a toolelectrode and a workpiece across a liquid-filled gap, the improvementwhich comprises a power supply:periodically operable electronic switchmeans including at least one first electronic switch having a controlelectrode and a pair of principal electrodes and a plurality ofparallel-connected second electronic switches each having a controlelectrode connected in circuit with the principal electrodes of saidfirst switch and a pair of principal electrodes connected in circuitwith said power supply and said gap for supplying said pulses thereto;periodically operable multivibrator means having an output connected tosaid control electrode of said first switch for triggering said switchmeans with preset on-off time, said multivibrator including a pair ofalternately conductive electronic switches and respectiveresistance-capacitance networks in circuit with each of said alternatelyoperable electronic switches, each of said resistance-capacitancenetworks having a respective time-constant-determining impedance forcontrolling on-off time; sensing means connected across said gap forproviding an electrical output signal responsive to a condition of saidgap; control means between said sensing means and at least one of saidimpedance and bridged thereacross for shunting the bridged impedance andaltering the effective impedance value and the time constant of thecorresponding resistance-capacitance network in accordance with saidsignal; and delay means between said sensing means and said controlmeans for delaying operation of said control means for a predeterminedtime interval after occurrence of said condition.
 11. The improvementdefined in claim 10 wherein:said multivibrator means is a transistormultivibrator; said alternately operable electronic switches aretransistors having bases tied to the respective resistance-capacitancenetworks; and the impedance bridged by said control means is a resistorof the corresponding resistance-capacitance network.
 12. The improvementdefined in claim 11 wherein said power supply includes:analternating-current source; a saturable reactor in series with saidsource; a rectifier in series with said saturable reactor and saidsource and connected between said saturable reactor and saidperiodically operable electronic switch means; and further control meansresponsive to the electrical current flow through said gap forcontrolling said saturable reactor.
 13. The improvement defined in claim11 wherein said control means includes:a pair of switching transistorseach having its principal electrodes connected across one of saidimpedances and respective control electrodes; an integrating networkconnected to the control electrodes of said switching transistors; and adifferentiating network connected between said delay means and saidintegrating network.
 14. In a method of electrical discharge machiningof a conductive workpiece with a tool electrode across a machining gapwherein a power supply is periodically connected to the machining gapthrough an electronic switch to provide machining pulses of apredetermined on-off time frequency to said gap and the dischargeincludes waveform harmonics of a higher frequency than that of saidmachining pulses and representative of a normal-gap cutting condition,the improvement which comprises the steps of sensing selected relativelyhigh-frequency harmonics representative of the normal-gap cuttingcondition across said gap; deriving an electrical control signal as afunction of the frequency level of said harmonics; and controlling thepulse on-off time with said signal.
 15. In an apparatus for electricallymachining a conductive workpiece by passage of electrical machiningpulses across a coolant-filled gap between a tool electrode and saidworkpiece, said apparatus comprising a machining power circuit forsupplying said pulses to said gap and including a power supply, aperiodically operated electronic switch of preset on-off time andprincipal electrodes connected in series with said power supply and saidgap, said electronic switch having at least one of its on-time andoff-time parameters controllable, the improvement which comprisessensing means continuously connected to said gap for providing anelectrical output signal continuously responsive to gap condition,control means between said sensing means and said switch for varying oneof said parameters in response to said signal, and delay means betweensaid sensing means and said control means for delaying its operation apredetermined time interval after occurrence of said abnormal gapcondition.
 16. The improvement defined in claim 15 wherein said controlmeans includes an integrating circuit network responsive to thecontinually sensed gap condition for varying one of said parameters..Iadd.
 17. An EDM apparatus of the type comprised in part of a powersupply producing periodic output machining pulses for generatingelectrical discharges across a machining gap formed between anelectrically conductive tool and an electrically conductive workpiece,wherein the improvement comprises:a. means connected to the tool andworkpiece for detecting the presence and absence of a noise signalduring the discharges; and b. means connected between the detectingmeans and the power supply for modifying the production of the machiningpulses in response to detecting the absence of the noise signal..Iaddend. .Iadd.
 18. An EDM apparatus of the type comprised in part of apower supply producing periodic output machining pulses for generatingelectrical discharges having a predetermined energy level across amachining gap formed in a dielectric medium between an electricallyconductive tool and an electrically conductive workpiece, wherein theimprovement comprises: (a) means electrically connected to the tool andworkpiece for detecting the absence of a noise signal during theelectrical discharges; (b) means connected between the detecting meansand the power supply for changing the energy level of the electricaldischarges in response to detecting the absence of said noise signal..Iaddend. .Iadd.
 19. An EDM process for removing metal having the stepsof initiating a spark discharge across a machining gap, melting a volumeof metal and terminating the spark discharge after a predeterminedperiod of time commensurate with a predetermined energy level, whereinthe improvement comprises: (a) detecting the absence of a noise signalacross the gap during the spark discharge; and (b) reducing the energylevel of succeeding spark discharges for a predetermined period of time..Iaddend.